It depends on how large a tank you have and what kind of filtering system you use. A good rule to use is for adults one inch of fish equals two gallons of water. Of course, when considering smaller fish you can crowd them up a bit more. The most important factor is surface area of the water, where the oxygen supply is being absorbed by the water at all times. In a typical 55 gallon tank you could have 6 fully adult discus or fifty small ones up to three inches in size. At 3 inches in size you will need to move half ( 25 ) of the fish to another 55 or larger tank. As those approach 4 inches in size you should again move half the fish ( 12 or 13 ) to another 55 gallon or larger tank. Between 4 1/2 to 5 inches you should be at 6 discus to a standard 55 gallon tank. All this assumes daily water changes at minimum 25% rate, but better is 45% water change.